The present invention relates generally to jet aircraft gas turbine engines which have fan blades, and more particularly to a nacelle which surrounds such fan blades.
A gas turbine engine includes a core engine having a high pressure compressor to compress the air flow entering the core engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a high pressure turbine which is rotated by the propulsive gas flow and which is connected by a larger diameter shaft to drive the high pressure compressor. A typical front fan gas turbine engine adds a low pressure turbine (located aft of the high pressure turbine) which is connected by a smaller diameter coaxial shaft to drive the front fan (located forward of the high pressure compressor) and which may also drive a low pressure compressor (located between the front fan and the high pressure compressor). The low pressure compressor sometimes is called a booster compressor or simply a booster. An engine casing, which surrounds the compressor and turbine blades, has a forward portion called a flow splitter. The flow splitter is located between the fan and the first (usually the low pressure) compressor to separate the air which exits the fan into a core engine airflow and a coaxially surrounding bypass airflow. The bypass airflow from the fan provides most of the engine thrust. Some of the engine thrust comes from the core engine airflow after it flows through the low and high pressure compressors to the combustor and is accelerated past the high and low pressure turbines and out the exhaust nozzle.
The front fan includes a fan nacelle which surrounds the fan blades and which is attached to the engine casing by hollow fan frame struts. The fan nacelle includes a generally cylindrical and longitudinally extending airfoil body having an inner surface which helps to channel the fan bypass airflow in a longitudinally aft direction for efficient production of engine thrust. The fan nacelle, like any object placed in an airstream, will produce unwanted drag. Drag is significantly decreased if the air not going through the fan nacelle can stay attached to the fan nacelle's outer surface while flowing past such outer surface. If such flow separates from the fan nacelle's outer surface, drag increases significantly. This happens during what is called engine windmilling, which occurs when that particular engine is not operating.
A known technique for reducing fan nacelle drag during engine windmilling is to make the fan nacelle thicker so that the airflow will go around less sharp of a turn at the front and not separate from the outer surface of the fan nacelle. Since it is undesirable to decrease the size of the fan blades, such a nacelle necessarily will have a larger diameter outer surface. However, this causes a substantial increase in fan nacelle size and weight. The increased weight of such a nacelle would itself lower aircraft performance. Also, some aircraft have wing mounted engines whose fan nacelles can't be made any larger and still clear the ground.